In the conventional type of derailleur gear shift mechanism on a bicycle, the rear wheel carries a stack of sprockets of different sizes. The drive chain from the pedal driven sprocket is engaged selectively with the rear wheel sprockets to provide the required gear ratio. The chain is held adjacent the rear sprockets on a shift carriage, which is moved laterally to transfer the chain from one sprocket to another as the sprockets rotate. A spring loaded idler sprocket on the shift carriage maintains tension in the chain to compensate for the different path lengths of the chain around the various sprockets.
The shift carriage is moved in one direction by a tension cable and is returned by a spring, the cable being connected to a shift lever on the upper front portion of the bicycle frame. In most installations there is no positive positioning means for the shift lever and shifting is done mostly by feel as the drive ratio changes. The mechanism is initially set up by adjusting the cable length, such as by a screw end fitting, and by setting various stops. Considerable skill and care is necessary to achieve precise alignment and periodic adjustment is usually required. The cable can bind in its guide tube due to corrosion or dirt, and the return spring can stick or fail to move the mechanism properly. The wide range of gear ratios obtained with the derailleur mechanism is very convenient, but it is recognized that constant maintenance is required to sustain proper operation.